This invention relates to an information processing system and, more particularly, to an information processing system that includes read-only-memory (ROM) units.
ROMs are nonvolatile memories that are often utilized in information processing systems. In such systems, the ROM memories are designed to perform a variety of functions including data conversion, table look-up, etc.
ROM units can be programmed in any one of various standard ways. Thus, for example, such units can be mask-programmed ROMs, field-programmable (e.g., fusible-link) ROMs, UV-erasable EPROMs or electrically-alterable EAROMs.
Heretofore, ROMs have been interconnected with other components of a processing system in various ways. Typically, the ROMs are connected to both address and data buses in the system. In such an arrangement, the ROMs constitute a portion of the program memory of the system and often entail in practice a considerable amount of instruction overhead. Moreover, such an arrangement of ROMs fragments program space and typically requires a relatively large amount of ROM to store a given amount of information.
Accordingly, efforts have been made by workers skilled in the art directed at trying to devise other ways of incorporating ROMs into a processing system. In particular, these efforts have been aimed at attempting to reduce the amount of ROM required to store a given amount of information and, at the same time, to improve the organization of program space in the system and to reduce the instruction overhead involved in accessing and reading ROMs. It was recognized that these efforts, if successful, could reduce the cost and enhance the mode of operation of processing systems including ROMs.